Method for removing arsenic and antimony from copper ore concentrates

ABSTRACT

Method for removing essentially all of the chemically bonded arsenic and antimony from copper sulfide ores. The arsenic and antimony are selectively leached out using Na2S in a basic solution. The resulting Na3AsS4 and Na3SbS4 are converted to the sulfides or to ferric arsenate and ferric antimonate for safe disposal. Sodium and sulfur values are recovered for recycling in the process.

United States Patent 11 1 Nadkarni et a1.

1 1 Oct.7, 1975 1 METHOD FOR REMOVING ARSENIC AND ANTIMONY FROM COPPERORE ('ONCENTRATES [75] Inventors: Ravindra M. Nadkarni, Arlington;

Charles L. Kusik; Herman P.

Mcissner, both of Winchester, all of Mass [73] Assignce: Arthur D.Little, lnc., Cambridge,

Mass

{22] Filed: Sept. 20, 1972 I21] Appli No.1290.442

[56] References Cited UNITED STATES PATENTS 706349 101905 MacArthur423/87 111245372 111/1918 Hooge ct ulm. 423/566 2.3298161 9/1943Bettcrton et al 423/87 1344,1114 3/1944 PCIHIE,.. H i i 423/566 3,107,9710/1963 Borrali et al. i i i i i 44 423/602 1709680 1/1973 Holmes 01 alt 75/6 OTHER PUBLICATIONS Remy, Treatise on Inorganic Chemistry, 1956,pg. 660, (Vol. 1). Agladze, Extraction of Arsenic from Sulfidic ArsenousOres, Tr, lnsta. Metalla i Gorn. Dela. An Gruzssr, 1957, Vol. 8, pp155-161. Holmes, General Chemistry, 1925, p. 382, Jacobson, Encyclopediaof Chemical Reactions. l95o Vol, 6, ppv 358359. Akademiianauk gruziuskoiUSSR-Instituta Metalla l Gornoso Dela Trudy-Vol. 8, 1957- pp. l55*|(w1Agladze et al.

Primary E.wrwzirmr-Oscar R4 Vertiz Assistant E.ranzinerEugene TiVvheelock Almrnc'y, Agent, or Firm-Schiller & Pandiscio [57] ABSTRACTMethod for removing essentially all of the chemically bonded arsenic andantimony from copper SUlfitl c ores. The arsenic and antimony areselectively leached out using Nai s in a basic solution. The resultingNa A854 and Na;,SbS are converted to the sulfides or to ferric arsenateand Ferric antimonatc for safe disposal. Sodium and sulfur values arerecovered for recycling in the process,

18 Claims, 6 Drawing Figures US. Patent Oct. 7,1975 Sheet 1 on 3,911,078

AQUEOUS LEACH LIQUOR CONTAINING N0 8 NuOH- LEACHING Cu AsS PREGNANTSOLUTION OF Nu As S +Nu S+No0H FURTHER TREATMENT Fig. I

PREGNANT SOLUTION OF Nu As S +No S+Nu OH CRYSTALLIZATION TO LEACHINGSTEP Nn S+No0H FILTRATION CRYSTALS OF Nc As S BH O AND FOR FURTHERTREATMENT Fig. 2

METHOD FOR REMOVING ARSF 'IC AND ANTIMONY FROM COPPER ORE CONCEN'IRA'IESThis invention relates to a method for treating copper ore concentrates.and more particularly to a method for removing arsenic and antimony fromcopper ore concentrates containing relatively large quantities of one orboth of these elements.

Several types of copper ore. particularly enargite. contain relativelylarge quantities of arsenic. e.g.. as much as l()-l 5% by weight and maycontain appreciable quantities of antimony. e.g., up to Z"? by weight.Both ofthese elements must be removed in the production of copper metal.and it is highly desirable that they be removed from the oreconcentrates prior to shipping to the smelter to eliminate smelterpenalties and to reduce shipping weight. Moreover. it is essential thatthe arsenic and antimony be removed by a method which minimizespollution control problems and produces waste products in a form inwhich they can be conveniently. economically and safely disposed of.Finally, the cost of removing these elements from copper oreconcentrates at or near the mine should not exceed the social andeconomic benefits resulting from the elimi nation of smelter penaltiesand from lowering the shipping costs.

It is therefore a primary object of this invention to provide animproved process for removing arsenic and antimony from copper oreconcentrates. It is another object to provide a method of the characterdescribed which is a nonpolluting hydrometallurgical procedure and whichconverts the arsenic and antimony into compounds which may be easily andsafely disposed of. An additional object is to provide a method forselectively removing arsenic and antimony from copper ore concentratesto leave a copper sulfide residue with a particle size distributiontypical of flotation concentrates. Still another object of thisinvention is to provide a method of the character described which doesnot re move appreciable amounts of valuable gold and silver values andwhich does not oxidize the sulfur in the copper ore concentrate tosulfur dioxide. It is a further ob ject to provide such a method whichmakes possible a choice of reactants for forming disposable arsenic andantimony compounds which may be either the sulfides or ferric arsenateand ferric antinionate. Other objects of the invention will in part beobvious and will in part be apparent hereinafter.

In accordance with this invention. arsenic (along with any antimonypresent) is removed from copper ore concentrates by leaching the orewith an aqueous solution of sodium sulfide containing sodium hydroxide.The undissolved copper sulfide in the form of parti' cles is removed byfiltering to leave a pregnant solution containing dissolved sodiumthioarsenate and sodium thioantimonate which are then converted to thesulfides or to ferric arsenate and ferric antimonate by one of severalreaction routes. The amount of sodium sul fide in the leaching liquid ispreferably in excess of the stoichiometric quantity required to reactwith all ofthe arsenic and antimony present in the copper oreconcentrate and the excess sodium sulfide is preferably recycled to theleaching step.

The following detailed description of the method of this invention willbe directed to the removal of arsenic from copper ore concentrates. Itis to be understood.

however. that the method is also applicable to the removal of anyantimony present by completely anal-a gous chemical reactions. Likewise.it is also applicable to ores which contain antimony as well as arsenic.It is not felt that it is necessary to write the corresponding equationsfor the reactions which involve antimony since they may be readilydeduced from those written for the reactions involving arsenic.

The invention accordingly comprises the several steps and the relationof one or more of such steps with respect to each of the others thereof.which will be exemplified in the method hereinafter disclosed, and thescope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of this invention,reference should be had to the following detailed description taken inconnection with the accompanying drawings in which FIG. 1 is a diagramof the first two steps of the method of this invention, i.e.. leachingand separation of the leach slurry into clean Cu S copper oreconcentrate and a pregnant solution containing dissolved arsenic andantimony;

FIG. 2 is a diagram illustrating the recovery of free Na- S in thepregnant solution by the crystallization of sodium thioarsenate andsodium thioantimonate from the pregnant solution:

FIG. 3 is a diagram illustrating the steps in the use of sulfuric acidto convert the arsenic in the thioarsenate crystals to arsenic sulfideand to recover Na S for recy cling;

FIG. 4 is a diagram illustrating the steps in the use of carbon dioxideabsorption to convert the arsenic in the thioarsenate crystals toarsenic sulfide and to recover Na S for recycling;

FIG. 5 is a diagram illustrating the steps in the use of sulfur dioxideabsorption to convert the arsenic in the thioarsenate crystals toarsenic sulfide. and to recover Na S for recycling; and

FIG. 6 is a diagram illustrating the conversion of the arsenic sulfideobtained by the methods shown in FIGS. 3, 4 and S to ferric arsenate.

A typical arsenic containing copper sulfide mineral may be assumed to beCu AsS wherein both the cop per and arsenic exist as the sulfide. Thefirst step of the method of this invention as diagrammed in FIG. I isbased upon the fact that sodium sulfide will dissolve sulfides ofarsenic and antimony. For arseniccontaining copper ore the solution ofarsenic is according to the reaction In the method of this invention.the first step of leaching out the arsenic. and any antimony. isaccomplished by contacting the ore concentrate with an aqueous basicsolution of sodium sulfide containing sodium hydroxide or other hydroxylion to prevent the formation of SH ions at the expense of S ions. Theconcentration of the aqueous solution may range from about [.5 to 3.5molar Na S and from about 0.2 to 1.0 molar NaOH or other hydroxyl ion. Apreferable leaching liq uid contains about 160 grams of Na S and about10 grams of NaOH per liter. equivalent to about 2 molar Na s and to 0.25molar NaOH. When an appreciable amount of antimony is present in thecopper ore concentrate (cg. more than about I)? by weight) then it maybe preferable to increase the concentration of the NaOH in the Nit- Sleach liquid up to as much as 3 molar.

The amount of Na S contained in the leach liquid should be equal to orpreferably in e\cess of the stoi chiometric quantity required to reactwith all of the arsenic and antimony present in the ore to form the thioarsenate and thioantimonatev The dissolution of arsenic from the ore isassumed to follow the equation which shows that 1.5 moles of sulfide arerequired per mole ofarsenic. It is preferable in leaching to use about 2to 3 times the stoichiometric quantity of sodium sulfide. or about 3 to4.5 moles of Na S for every mole of arsenic and every mole of antimonypresent. The pres-- ence of this excess Nil- S assures essentiallycomplete removal of the arsenic and antimony. The upper stoichiometricratios of Na S and the concentration of Na. .S in the leach liquid maybe influenced by the possi bility of the oxidation of the M 5 duringleaching and should therefore be maintained at levels below which anyappreciable oxidation occurs.

Leaching is preferably carried out at an elevated temperature betweenabout 80C and I06C and for a time to complete the dissolution of thearsenic and antimony present in the ore concentrate particles. At theseelevated temperatures and atmospheric pressure, this dissolution isnormally complete in several hours. eg. in about 2 to 5 hours. It isperferable to provide the ore concentrate in the form of particlesranging in size from about HM) to Mitt-mesh.

Subsequent to the dissolution of the arsenic and antimony. as detailedin reaction (I). the copper sulfide particles in the leach slurry.essentially free of arsenic and antimony but still containing most orall of their original gold and silver values, are separated from thepregnant solution by filtration and dried. In this condition andcontaining less than one percent by weight of arsenic and antimony theymay be reduced to copper by any suitable smelting process. Filtration ofthe leach slurry is preferably carried out at a temperature be tweenabout (10C and 106C.

Because the arsenic in the pregnant solution is in the form of solublethioarsenate. AsS, I it is necessary to convert it chemically to aninert. insoluble form for safe disposal. Of all of the arsenic compoundsknown. the arsenic sulfides. AsS. As. .S or AS255. and ferric arsenate(Scorodite. FeAsO .2H O) are considered to be the least toxic and mostinert. i.c., least susceptible to being dissolved to form awater-soluble contaminant. Therefore. the step of removing the arsenicfrom the pregnant solution is directed to the formation of arsenicsulfide. hereinafter represented as AS285, or FeA- s0 Likewise. the stepof removing the antimony from the pregnant solution is directed to theformation of an timony sulfide or ferric antimonate.

Since an excess amount of sodium sulfide is present in the leach liquorand is therefore transferred to the pregnant solution. it is highlydesirable from an economic point of view to be able to recover thissodium sulfide prior to the conversion of the sodium thioarsenate to asafe disposable form and to recycle it to the leaching step. It is alsohighly desirable to recover the sodium values from the product sodiumthioarsenate in the form of Na S for recycling.

The free Na s in the pregnant solution is recovered by crystallizing outthe sodium thioarsenate and sodium thioantimonate by well-known methodssuch as evaporation or cooling. The crystals are separated by filteringand the N1 8 solution is returned to the leaching step. HQ. 2 is adiagram ofthis series of steps. After the free Na S in the pregnantsolution is recovered in this manner. one of the following three seriesof steps can he used to rccov er the combined sodium sulfide from thethioarsenate and thioantimonate and to convert these compounds toinsoluble forms such as arsenic sulfide and antimony sulfide or ferricarsenate and ferric antimonate.

FIG. 3 is a diagram of the steps employed in the removal of arsenic froma solution of sodium thioarsenate through acidification with a strongmineral acid. Although it is possible to use one of several strongmineral acids. e.g.. sulfuric. hydrochloric. nitric and phosphoric.sulfuric acid is preferred since it is usually the cheapest acidavailable. and is used as exemplary in the following description as wellas in FIG. 3.

In the acidification of the pregnant solution the desired arseniccompound As S is precipitated out through the mechanism representable byequation (3) In order to remove essentially all of the arsenic from thepregnant solution as As S it is necessary to add sufficient sulfuricacid to lower the pH of the reaction system liquid to 5 and preferablyto 4 or below. With the completion of H 5 evolution, the pH may thendrop to about 1. Although the concentration of the sulfuric acid addedis not critical, it is preferred to use a rela tively concentrated acid.e.g.. 3 normal or above.

As will be seen in the diagram of FIG. 3, the As S in the form of alemon-yellow precipitate, is then filtered out and disposed of in asuitable manner. such as by burying. The filtrate, which containsproduct sodium sulfate in solution can be treated in one of threealternative ways to recover the the sodium values pres ent in the sodiumsulfate. These alternative routes are designated as (l). (2) and (3) inFIG. 3.

The first alternative route for the treatment of the Nil- S0. involvesthe reaction with barium sulfide t( form barium sulfate and sodiumsulfide and the recov cry of the barium values barium sulfide forrecycling.

Barium sulfide and sodium sulfate undergo a double decompositionreaction Na sO BaS BaSON N3 5 with the barium sulfate precipitating outessentially quantitatively. Filtration separates out the solid B2150from the resulting Na- S solution. the latter being suit able for use asleach liquid. The barium values are then recovered by reacting thebarium sulfate with carbon in a reduction kiln under known conditionswhich in clude a reduction temperature of about 700-1000C and 400 poundsof carbon per ton of BaSO 6" Any makeup barium values can be added inthe form of natural barite (82150,) to the reduction kiln. The BaSsolution is obtained by dissolving the kiln residue in hot water. and isthen recycled. The conditions which give rise to these reactionsinvolved in this first alternative way of recovering sodium values fromthe sodium sulfate are all well known to the inorganic chemist and neednot be detailed.

The second alternative route for the recovery of the sodium values as NaS involves the precipitation of calcium sulfate by the reaction Na SOCa( OH CaSO, l, ZNaOH. (in

After the removal of the calcium sulfate by filtering, the sodiumhydroxide solution is reacted with a portion of the hydrogen sulfidegenerated in the acidification of the pregnant solution ZNaOH H S Na SZZH Ov Before recycling these reaction products to the leach liquid. itmay be desirable to concentrate the sodium sulfide solution. As in thefirst alternative route, the conditions for the inorganic reactionsinvolved in this I second route are wellknown and are therefore notdiscussed in detail.

The third alternative series of steps for recovering the sodium valuesas Na s involves the crystallization of Na SO, from the solution and thesubsequent reduction of the Na SO, with carbon by the reaction Na SO 2CNa s ZCO (a As in the previous two alternatives, the conditions for thisroute are well known and are therefore not dis As will be shown in FIG.4 and illustrated in the examples which follow, a portion of the sodiumthioarsenate remains in solution and is carried through the steps forthe recovery of sodium values as sodium sulfide. This unprecipitatedsodium thioarsenate is then returned along with the recovered sodiumsulfide to the leach solution.

Sodium bicarbonate can then be extracted from the mixed precipitate byheating with steam which converts the bicarbonate to soluble carbonateZNaHCO;, Nat- C0 H O CO (Ill The (O is available for recycling while theNa CQ; is

treated with lime The sodium hydroxide is then reacted with the hydrogensulfidc formed in the CO treatment step to form Na s and water accordingto reaction (7) above.

The third embodiment of the treatment of the thioarsenate solution.which is diagrammed in FIG. 5, is also a form of acidification. using S0It is well known that SO dissolves in vvater to form sufurous acid, H 50which is a much stronger acid than H CO Therefore, 6

the reaction which occurs when SO gas is introduced into the sodiumthioarsenate solution may be represented as lNa Ass, 3H SO 3Na SO; As Sl, 3H S.

I I3 l After filtration of the precipitated A5 5 the filtrate is treatedwith lime to precipitate calcium sulfite and form sodium hydroxide. Thesodium hydroxide is reacted with hydrogen sulfide and the sodium isrecovered as Na S as shown in reaction (7).

An alternative approach is to oxidize the sodium sulfite to sodiumsulfate with air N21 50:; V2 0 Na SC) 14) and then utilize one of thethree methods illustrated in FIG. 3 for conversion of Na SO, to Na s.

Sulfur dioxide must be introduced until a pH of about 5 is attained. Dueto the fact that a portion of the H 5 which is formed is converted tofree suflur by the reaction 2H S SO 35 2H O, (l5) complete precipitationof the arsenic as As S cannot be accomplished with S0 alone. Therefore,if it is nccessary to remove essentially all of the arsenic from thesodium thioarsenate solution, it is necessary to add H 8 to the finalacidic solution and precipitate As- S as is practiced in qualitativeanalysis.

The arsenic sulfide produced by any one of these three methodembodiments described for sodium thioarsenate treatment may be convertedto ferric arse nate, FeAsO as the disposable arsenic compound asdiagrammed in FIG. 6. The precipitated As S is acid pressure leached(typically under a pressure of from about l0 to 200 psig) in anautoclave As s BH O 2H AsO SH SO m The resulting acid mixture ispartially neutralized by addition of lime H SO Ca(OH) CaSO H 0 117 toeffect pH control. Subsequently to or concurrently therewith ferricsulfate is added to convert the arsenic to the insoluble ferric arsenateor Scorodite 2H AsO Fe (SO 2FeAsO 3H;,SO no The sulfuric acid may berecycled for use in acidification of the sodium thioarsenate solution.

The following examples, which are meant to be illustrative and notlimiting, are given to further describe this invention.

The copper ore concentrate used was undried enar gite which contained 9%water by weight and l 1% arsenic by weight on a dry basis. In all of theexamples 54.5 gms of this ore concentrate. in the form of particlesranging in size from 100- to 400-mesh was leached with I50 ml of a leachliquid which contained 30 gms of Na S (about 2.5 molar) and [.5 gm NaOH(0.25 mo lar). The amount of Na S in the leach liquid was three timesthe stoichiometric quantity required to react with the arsenic present.Leaching was carried out in a 250- ml round bottom flask fttted with amechanical stirrer. thermometer and access port for removing samples.The flask was controllably heated and a watercooled condenser wasinstalled to permit refluxing the enargite slurry at the boiling pointof the leach solution. The flask was charged with the weighed amount ofenargite Concentrate and the requisite amount of sulfide solution. Heatwas then applied to bring the solution to a boil 106C). and once boilinghad begun was reduced to maintain a slow, rolling boil throughout theduration of the extraction. Stirring with an electrically drivenmechanical stirrer was provided to ensure that the reaction mixture wasa well-mixed slurry. After the onset of boiling periodic samples of theslurry were withdrawn, filtered immediately and the filtrate analysedfor dissolved arsenic by standard atomic absorption spectrometrictechniques capable of minimum detectability in the range of l0 ug/ml.Knowing the complete removal of the arsenic from the 0.073 moles of Asin the 545 gram sample would amount to 37 mg As/ml in the leach liquormade it possible to determine the extent of arsenic removal from the oreconcentrate. The samples of the slurry which were withdrawn from thereactor were filtered through Whatman No. 4l paper and the filtrate wasretained for arsenic measurement. At the end of the experiment, theslurry remaining in the reactor was vacuum filtered and sucked as drypossible. The collected filtrate volume was noted. At the end of 2hours, over 90% of the arsenic had been removed and at the end of 4hours virtually all (over 9892 had been transferred to the leach liquorvIn this laboratory-scale run. when the amount of Na S in the 150 ml ofleach liquid was reduced to the stoichiometric quantity required toreact with the arsenic content of the ore, only about 13% of the arsenicwas removed after 5 hours. In large-scale industrial equipment it isexpected that more efficient mixing and contacting could be expected toincrease this percentage of arsenic removal while using less Na- S.Increasing the NaOH content by a factor of in the leach liquidcontaining this stoichiometric amount of Na s increased arsenic removalto about 62% after 5 hours of leaching. Thus it will be seen that theamount of Na s and NaOH relative to the amount of arsenic present has adirect bearing on the efficiency of arsenic removal and the timerequired to effect it.

Additional runs were made as described above to determine the effect ofvarying the sodium sulfide concentration in the leach liquid on arsenicextraction. Since little incremental benefit is obtained usingconcentrations above about 2.5 molar sodium sulfide, it is preferable tokeep the sodium sulfide concentrations as low as possible while stillaccomplishing arsenic removal. For example. if there is a possibilitythat sodium sulfide may oxidize slowly over time, presumably theoxidation wili be faster with higher concentrations of sodium sulfide.Also, at lower concentrations, the amount of occluded sodium sulfidegoing out with the filter cake will tend to be lower; thus, losses willbe min imized at lower concentrations. For any given type of equipment,condition of mineral particles (e.g.. wettability mixing efficiency etc.it is within the skill of the art to determine optimum concentrations,

When sodium hydroxide concentration is below about 0.25 molar there maybe sufficient hydrolysis of the sulfide ion to the hydrosulfide ion toprevent comlete reaction of the sodium sulfide with the arsenic inenargite.

Although it may be preferable to use the Na S in stoi chiometric ratiosof three times that required to obtain removal of essentially all of thearsenic. additional 6X perimental results have shown that more than 90percent arsenic extraction can be attained in reasonable leaching timesof 3 or 4 hours with a stoichiometric ratio of two times the amountneeded to complete the following reaction:

The stoichiometric ratio chosen will therefore be that required toattain a desired balance between such factors as reaction time. thepercent of arsenic removal desired and the possibility of oxidation ofthe sodium sulfide in the leach.

As the leaching is continued and the Na s solution derived from thepregnant solution as filtrate is recycled over and over again (carryingwith it minor amounts of dissolved arsenic and antimony in the form ofsodium thioarsenate and sodium thioantimonate) it may be necessary toincrease leaching time by a small amount to insure removal of apredetermined amount, e.g., 9092, of the arsenic from the oreconcentrate.

Leaching may be carried out in multiple steps and even in acountercurrent system if desired. Enargite was leached 1.5 hours using(2.5M Na S, a stoichiometric ratio of 2; 0.25M NaOH) to about 88%arsenic removal. After filtering the concentrate was then subjected to afurther treatment with a duplicate fresh leach solution. Preferably thisfresh leach solution is obtained from the Na S regeneration steps.Arsenic concentrations in the copper sulfide residue were reduced to0.50.6"/( and antimony to 0.04 to 0.05%. Since the ratio of arsenic toantimony was about the same in head and tail samples, it appeared thatthe antimony was leached out at about the same rate as arsenic underthese conditions.

Analyses of a number of pregnant liquors, prior to the crystallizationof the N83ASS4 indicated from about 1 to l071 of the gold content andsome 5 to 6% of the silver content of the total amount present in theore concentrate may be dissolved in the Na S leaching. A major quantityof this gold and silver may be recovered from the Na S solution prior torecycling.

Sodium thioarsenate was found to have a very steep solubility curve. Byleaching and then filtering off the pregnant solution at temperatures ashigh as practical and preferably between about 60C and the boiling pointl00l06C) of the leaching solution, the product sodium thioarsenateremains in solution during leaching and filtering. It is. however,readily crystallized out from the pregnant solution by cooling with orwithout seeding. Due to the steep solubility curve, crystallization ofsodium thioarsenate and sodium thioantimonate is readily accomplished inthis manner.

In the following examples illustrating the further processing of thethioarsenates to form arsenic sulfide or ferric arsenate, the productsodium thioarsenate and sodium thioantimonate were crystallized out ofthe pregnant solutions by cooling to about 40C. The resultingthioarsenate crystals (and thioantirnonate crystals where present) weredissolved in water to form a sodium thioarsenate solution forprocessing.

Separate sodium thioarsenate solutions thus obtained were then subjectedto the various combination of steps described for precipitation of thesulfides. The first solution sample was titrated with approximately 3N HIndicator papers were used to measure pH as the titration preceded. Theinitial sodium thioarsenate solution has a pH l2 indicating the presenceof some 5 OH and perhaps SH. During a preliminary neutralization ofthese ions the pH remained at the original high level. However, withcontinuing addition of acid the pH dropped quickly to between 7 and 8, H5 evolution was detected and a first lemon-yellow permanent precipitatewas formed. As the titration was continued. the pH dropped slowly fromabout 7 to about with the continuing. e\olution of H 5 and precipitationof the lemon vellow precipitate. Finally. gas evolution andprecipitation ceased. the precipitate agglomerated and the pH droppedquickly to approximately 1. After titration. the acidified solution wasfiltered and the filtrate was analyzed for dissolved arsenic. Nodetectable arsenic 5ug/ml) was found. indicating that essentially all ofthe arsenic had been removed the sulfide.

Another sample of the sodium thioarsenate solution was treated withgaseous CO A o-gram portion of Na AsS 8H O (equivalent to about 096 gmsarsenic) was dissolved in ml of water and placed in a ZOO-m] pressurebottle. Then the bottle was connected to a carbon dioxide tank. flushedwith carbon dioxide gas. and tinally pressurized to l5 psig and a purgerate of about 1 liter per minute established. After reacting (withstirring) for l hour under those conditions. the solution was observedto be deep yellow in color. the odor of hydrogen sulfide could easily bedetected in the gaseous effluent. but no precipitate was observed. Thepressure was then increased to psig. the flow rate adjusted to about lliter per minute, and the reaction allowed to proceed for l6 hours. Atthe end of this time. some yellow-brown precipitate was observed in thereaction flask. The flask was opened. the precipitate filtered off. andits weight determined to be about 0.12 gram (equivalent to 0.06 gramarsenic if the precipitate were As S,-,]. The filtrate was analyzed forsoluble arsenic by atomic absorption and was found to contain a total0.9l grams of arsenic. Thus. reaction overnight with carbon dioxide at30 psig precipitated about 567( of the initial arsenic and 95% was foundto remain in solution.

At the conclusion of the 16 hour experiment mentioned above. the pH ofthe solution was determined to be about 6 immediately after the vesselwas opened. But upon opening. a good deal of etfervescence was noted andthe pH was observed to rise rather rapidly up to a value of about 8.

The fact that in this experiment the pH could not be brought below thatpoint where a major portion of the arsenic could be converted to As s;was attributed to the buffering behavior of the large amount (on theorder of l molar) of sodium bicarbonate formed. Thus in using CO toprecipitate the arsenic it might be necessary to supplement the actionof the H CO with an additional strong acid such as H SO or to recyclethe arsenic containing solution as shown in FIG. 4.

Using a third sample of a sodium thioarsenate crystals. SO wassubstituted for (0 Six grams of hydrated Na AsS, (equivalent to about0.96 gram arsenic) was dissolved in 30 ml of water. Sulfur dioxide atatmo sphcric pressure was bubbled through this solution at a rate of It)ml per minute. Sulfur dioxide absorption was obscn ed to be quite rapid.particularly when com pared to the absorption of carbon dioxide. Afterflowing for only a few minutes a precipitate was observed in thesolution and the pH dropped to about 6. The pre cipitate was filteredand its dry weight determined to be 0.04 grams. The filtrate was thenreturned to the reactor and the introduction of sulfur dioxide wascontin ued until the solution pH had fallen to about 2. During thissecond step. additional precipitate was observed. and at the completionof the step. it was filtered and weighed. Some solids were observed topass through the filter into the filtrate. The residue obtained afterfiltration of the final reaction mixture was dried and its weightdetermined to be L34 grams. If all the arsenic in solution had beenprecipitated. the precipitate would have weighted 1.57 g or L99 g.depending on whether the precipitate was As S or A5 8 Since theprecipitate weighed less than this. less than complete elimination ofarsenic is indicated.

A sample of the dried solids was analyzed for sulfur content and thiswas determined to be 48.2% This suggests a mixture of sulfides. As S andAs. ,S with perhaps some free sulfur.

If total arsenic removal is desired. then H 5 gcncrated in the reactionbetween the sodium thioarsenate and sulfur dioxide may be bubbledthrough the pH 6 solution to precipitate all of the arsenic as As- S asis practiced in qualitative analyses.

It is therefore possible by the method of this invention to extractessentially all of the arsenic. and any antimony with it. from coppersulfide ore concentrates and to form the arsenic and antimony thusextracted into compounds suitable for direct disposal. The arsenic-freeore concentrate is in a desirable particle size range for furtherprocessing to extract copper without introducing any arsenic pollutantsinto the atmosphere. Moreover. most of the precious metals (silver andgold) remain in the concentrate for recovery by standard procedures.

It will thus be seen that the objects set forth above. among those madeapparent from the preceding description are efficiently attained and.since certain changes may be made in carrying out the above methodwithout departing from the scope of the invention. it is intended thatall matter contained in the above description shall be interpreted asillustrative and not in a limiting sense.

We claim:

1. A method of treating an enargite copper ore containing arsenic andantimony to remove the arsenic and antimony therefrom. comprising thesteps of a. leaching copper enargite containing chemically combinedarsenic and antimony with an aqueous leach solution of sodium sulfide.present in a con centration ranging between about 1.5 and 35 mo lar.containing hydroxyl ions in a concentration of between about 0.2 and 3molar. at a temperature between about C and the boiling point of saidleach solution for a time sufficient to dissolve out essentially all ofsaid arsenic and antimony and form sodium thioarsenate and sodiumthioantimonate. the amount of said leach solution being that which willprovide said sodium sulfide in a quantity between about two and threetimes the stoichiometric quantity required to react with all of the arsenic and antimony present in said ore concentrate; filtering theresulting leach slurry at a temperature between about 60 and about C toremove the arsenic and antimony-free copper ore concentrate from theleach liquor and to give a filtrate in the form ofa pregnant solutioncontaining said sodium thioarsenate and sodium thioantimonate dissolvedtherein; removing said sodium thioarsenate and sodium thioantimonatefrom said pregnant solution by crys tallization and subsequentfiltration; and d. subjecting said sodium thioarsenate and sodiumthioantimonate to an acid treatment thereby to precipitate said arsenicand antimony as arsenic sulfide and antimony sulfide.

2. A method in accordance with claim 1 including the step of recoveringsodium sulfide from the liquor re sulting from said acid treatment.

3. A method in accordance with claim 1 wherein said hydroxyl ions areprovided sodium hydroxide.

4. A method in accordance with claim 1 wherein said crystallization ofstep (c) comprises cooling said pregnant solution.

5. A method in accordance with claim 1 wherein said step of subjectingsaid crystals to an acid treatment comprises reacting said sodiumthioarsenate and said sodium thioantimonate with sulfuric acid to reducethe pH to about 5 thereby to precipitate the sulfides of arsenic andantimony and form sodium sulfate and hydrogen sulfide.

6. A method in accordance with claim 5 including the step of dissolvingsaid crystals in water prior to reacting said sodium thioarsenate andsodium thioantimonate with said sulfuric acid.

7. A method in accordance with claim 5 further including the steps offiltering out the precipitated sulfides of arsenic and antimony to forma filtrate containing said sodium sulfate and converting said sodiumsulfate to sodium sulfide for recycling as at least a portion of saidaqueous leach solution.

8. A method in accordance with claim 7 wherein said step of convertingsaid sodium sulfate to sodium sulfide comprises reacting said sodiumsulfate with barium sulfide to form sodium sulfide and barium sulfate.

9. A method in accordance with claim 8 including the further step ofreducing the product barium sulfate with carbon to form barium sulfidefor reaction with said sodium sulfate.

10. A method in accordance with claim 7 wherein said step of convertingsaid sodium sulfate to sodium sulfide comprises reacting said sodiumsulfate with calcium hydroxide to form sodium hydroxide and calciumsulfate and regenerating said sodium sulfide by reacting said sodiumhydroxide with said hydrogen sulfide formed in said acid treatment ofsaid sodium thioarsenate and sodium antimonate.

1]. A method in accordance with claim 7 wherein said step of convertingsaid sodium sulfate to sodium sulfide comprises reducing said sodiumsulfate by reaction with a reducing agent.

12. A method in accordance with claim 7 including the steps of acidpressure treating said sulfides of arsenic and antimony under oxygen toform a mixture of arsenic. antimonie and sulfuric acids, partiallyneutralizing said acid mixture to form a sulfate salt and reacting saidarsenic and antimonic acids with ferric sulfate to form insoluble ferricarsenate and ferric antimonate.

13. A method in accordance with claim 1 wherein said step of subjectingsaid sodium thioarsenate and sodium thioantimonate to an acid treatmentcomprises bubbling carbon dioxide gas through an aqueous solution ofsaid sodium thioarsenate and sodium thioantimonate until the pH of saidsolution levels off at a minimum value thereby converting a portion ofthe arsenic and antimony to the sulfides.

14. A method in accordance with claim 13 including the step of addingsulfuric acid to said solution to complete the precipitation of thearsenic and antimonic sul- 15. A method in accordance with claim 1wherein said step of subjecting said sodium thioarsenate and said sodiumantimonate to an acid treatment comprises bubbling sulfur dioxide gasthrough an aqueous solution of said sodium thioarsenate and sodiumantimon ate until a pH of about 2 is attained thereby forming sulfidesof arsenic and antimony, sodium sulfite and hydrogen sulfide.

16. A method in accordance with claim l5 including the step of bubblingsaid hydrogen sulfide through said aqueous solution after attainment ofsaid pH of about 2 thereby to complete the precipitation of saidsulfides of arsenic and antimony.

17. A method in accordance with claim 15 including the further steps ofconverting said sodium sulfite to so dium hydroxide and thenregenerating sodium sulfide from said sodium hydroxide by reacting itwith said hydrogen sulfide formed in said acid treatment.

18. A method in accordance with claim 15 including the steps ofoxidizing said sodium sulfite to sodium sul fate and then regeneratingsodium sulfide from said sodium sulfate.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,9ll,078

DATED I October 7 1975 INVENTOR( RAVINDRA M. NADKARNI ET AL It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 10, line 62 should read:

c. crystallizing said sodium thioarsenate and said sodium thioantimonatein said pregnant solution to form a recycle liquid containing sodiumsulfide and hydroxyl ions;

d. separating said precipitated sodium thioarsenate and said sodiumthioantimonate from said recycle liquid by filtration;

e. using said recycle liquid directly in said leaching step; and

f. subjecting said sodium thioarsenate and sodium thioantimonate to anacid treatment thereby to precipitate said arsenic and antimony asarsenic sulfide and antimony sulfide.

Signed and Scaled this second Day of March 1976 [SEAL] A ttesr:

RUTH C. MA'SON V C. MARSHALL DANN Alteslmg Of frcer CommissionerojParenls and Trademarks

1. A METHOD OF TREATING AN ENARGIC COPPER ORE CONTAINING ARENIC ANDANITIOMY TO REMOVE THE ARSENIC AND ANTIMONY THEREFROM COMPRISING THESTEPS OF A. LEACHING COPPER ENERGATIC CONTAINING ACHEMICALLY COMBINEDARSENIC AND ANTIMONY WITH AN QUEOUS LEACH SOLUTION OF SODIUM PRESENT INACONCENTRATION RANGING BETWEEN ABOUT 1.5 AND 3.5 MOLAR, CONTAININGHYDROXYL IONS IN A CONCENTRATION OF BETWEEN ABOUT 0.2 AND 1.3 MOLAR, ATA TEMPERATURE BETWEEN ABOUT 80*C AND THE BOILING POINT OF SAID LEACHSOLUTION FOR AFIME SUFFICIENT TO DISSOLVE OUT ESSENTIALLY ALL OF SAIDARESENIC AND ANTIONY AND FROM SODIUM THIOARAENATE AND SODIUMTHIOANATIMOATE, THE AMOUNT OF SAID LEACH SOLUTION BEING THAT WHICH WILLPROVIDE SAID SODIUM SULFIDE IN A QUANTITY BETWEEN ABOUT TWO AND THREETIMES THESTICHIOMERIC QUALITY REQUIRED TO REACT WITH ALL THE ARESENTICAND ANITONY PRESENT IN SAID ORE CONCENTRATE, B. FILTERING THE LEACHSLURRY AT A TEMPERATURE BETWEEN ABOUT 60*C AND ABOUT 100*X TO REMOVE THEARENIC AND ANTIMONY-FREE COPPERR ORE CONCENTRATE FROM THE LEACH LIQUORAND TO GIVE A FILTRATE IN THE FORM OF A PREGNANT SOLUTION CONTAININGSAID SODIUM THIOARSENATE AND SODIUM THIOANTIMOMATE DISSOLVED THEREIN, C.REMOVING SAID SODIUM THIOARASENATE AND SODIUM THIOANTIMONATE FROM SAIDPREEGNANT SOLUTION BY CRYSTALLIZATON AND SUBSEQUUENT FILTRATION, AND D.SUBJECTING SAID SODIUM THIOARSENATE ANDSODIUM THIOANTIMONATE TO AN ACIDTREATMENT THEREBY TO PRECIPITATE SAID ARSENIC AND ANTIMONY AS ARSENICSULFIDE AND ANTIMONY SULFIDE.
 2. A method in accordance with claim 1including the step of recovering sodium sulfide from the liquorresulting from said acid treatment.
 3. A method in accordance with claim1 wherein said hydroxyl ions are provided as sodium hydroxide.
 4. Amethod in accordance with claim 1 wherein said crystallization of step(c) comprises cooling said pregnant solution.
 5. A method in accordancewith claim 1 wherein said step of subjecting said crystals to an acidtreatment comprises reacting said sodium thioarsenate and said sodiumthioantimonate with sulfuric acid to reduce the pH to about 5 thereby toprecipitate the sulfides of arsenic and antimony and form sodium sulfateand hydrogen sulfide.
 6. A method in accordance with claim 5 includingthe step of dissolving said crystals in water prior to reacting saidsodium thioarsenate and sodium thioantimonate with said sulfuric acid.7. A method in accordance with claim 5 further including the steps offiltering out the precipitated sulfides of arsenic and antimony to forma filtrate containing said sodium sulfate and converting said sodiumsulfate to sodium sulfide for recycling as at least a portion of saidaqueous leach solution.
 8. A method in accordance with claim 7 whereinsaid step of converting said sodium sulfate to sodium sulfide comprisesreacting said sodium sulfate with barium sulfide to form sodium sulfideand barium sulfate.
 9. A method in accordance with claim 8 including thefurther step of reducing the product barium sulfate with carbon to formbarium sulfide for reaction with said sodium sulfate.
 10. A method inaccordance with claim 7 wherein said step of converting said sodiumsulfate to sodium sulfide comprises reacting said sodium sulfate withcalcium hydroxide to form sodium hydroxide and calcium sulfate andregenerating said sodium sulfide by reacting said sodium hydroxide withsaid hydrogen sulfide formed in said acid treatment of said sodiumthioarsenate and sodium antimonate.
 11. A method in accordance withclaim 7 wherein said step of converting said sodium sulfate to sodiumsulfide comprises reducing said sodium sulfate by reaction with areducing agent.
 12. A method in accordance with claim 7 including thesteps of acid pressure treating said sulfides of arsenic and antimonyunder oxygen to form a mixture of arsenic, antimonic and sulfuric acids,partially neutralizing said acid mixture to form a sulfate salt andreacting said arsenic and antimonic acids with ferric sulfate to forminsoluble ferric arsenate and ferric antimonate.
 13. A method inaccordance with claim 1 wherein said step of subjecting said sodiumthioarsenate and sodium thioantimonate to an acid treatment comprisesbubbling carbon dioxide gas through an aqueous solution of said sodiumthioarsenate and sodium thioantimonate until the pH of said solutionlevels off at a minimum value thereby converting a portion of thearsenic and antimony to the sulfides.
 14. A method in accordance withclaim 13 including the step of adding sulfuric acid to said solution tocomplete the precipitation of the arsenic and antimonic sulfides.
 15. Amethod in accordance with claim 1 wherein said step of subjecting saidsodium thioarsenate and said sodium antimonate to an acid treatmentcomprises bubbling sulfur dioxide gas through an aqueous solution ofsaid sodium thioarsenate and sodium antimonate until a pH of about 2 isattained thereby forming sulfides of arsenic and antimony, sodiumsulfite and hydrogen sulfide.
 16. A method in accordance with claim 15including the step of bubbling said hydrogen sulfide through saidaqueous solution after attainment of said pH of about 2 thereby tocomplete the precipitation of said sulfides of arsenic and antimony. 17.A method in accordance with claim 15 including the further steps ofconverting said sodium sulfite to sodium hydroxide and then regeneratingsodium sulfide from said sodium hydroxide by reacting it with saidhydrogen sulfide formed in said acid treatment.
 18. A method inaccordance with claim 15 including the steps of oxidizing said sodiumsulfite to sodium sulfate and then regenerating sodium sulfide from saidsodium sulfate.